1. Field of the Invention
The invention relates to a vehicular steering system used for vehicles such as automobiles.
2. Description of the Related Art
As vehicular steering systems, what is called steer-by-wire (SBW) vehicular steering systems are known in which a steering member and a steering operation mechanism are not mechanically coupled together. For example, Japanese Patent Application Publication No. 2014-218192 (JP 2014-218192 A) describes a steer-by-wire vehicular steering system in which a steering wheel serving as a steering member and a steering operation mechanism can be mechanically coupled together via a clutch. The clutch is normally released, and steering operation control is performed in a steer-by-wire mode (SBW mode). If any abnormality occurs while steering operation control is being performed in the steer-by-wire mode, the clutch is engaged and the steering operation control is performed in an electric power steering (EPS) mode.
The applicant has proposed a clutch that is suitable for mechanically coupling and decoupling the steering wheel to and from the steering operation mechanism. The clutch proposed by the applicant includes an input shaft, an output shaft, an inner ring, and an outer ring. The inner ring is coaxially and integrally coupled to the input shaft. The outer ring is coaxially and integrally coupled to the output shaft. The clutch proposed by the applicant further includes a clutch mechanism and a solenoid actuator. The clutch mechanism transmits and blocks a torque between the inner ring and the outer ring. The solenoid actuator has a function to generate an electromagnetic force that allows the clutch mechanism to be released. The solenoid actuator thus allows the clutch mechanism to be engaged and released.
When the clutch used for vehicular steering systems as described above is released from the engaged state, an excessive engaging force (frictional force) may be being applied to the clutch, for example, by operating the steering wheel. If an excessive engaging force is being applied to the clutch proposed by the applicant when the clutch is released from the engaged state, the frictional force, which prevents the release of the clutch, is increased, leading to the need for a high electromagnetic force exerted to release the clutch. Thus, an electromagnet with high output is needed, disadvantageously resulting in an increased size of the clutch and increased power consumption.